This invention relates to carbon black having excellent electrical properties, particularly excellent magnetic properties, a process for preparing the same, and a method of modifying magnetic carbon black.
Hitherto, carbon black has been widely used as a rubber reinforcing filler in the field of tires and various industrial rubber products as well as in the fields of colorants of a printing ink, a paint, etc. and functional materials.
Since carbon black comprises carbonaceous matter and has a certain degree of electrical conductivity, it is also used as a filler for the purpose of imparting electrical conductivity. However, carbon black in itself has no magnetic performance.
Therefore, if the magnetic performance could be imparted to carbon black, magnetic rubbers, magnetic resins, etc. would be produced by imparting to composite rubbers and resins a magnetic function in addition to various functions inherent in carbon black, such as reinforcing power, tinting power, and electrical conductivity. These products are expected to be useful in the fields of electronic, mechanical and chemical industries.
Further, in the fields of inks and pigments as well, it is expected that carbon black is advantageously used for applications, such as magnetic ink, magnetic pigment, and toner for electrophotography, without spoiling blackness inherent in carbon black.
There are various kinds of carbon black according to particle diameters. In general, carbon black has a chain structure comprising spherical particles having an average particle diameter of 10 to 200 nm which have been fused and secondarily agglomerated.
Also the particle diameters have a wide distribution according to the average particle diameter
Further, the density of carbon black is about 1.8, i.e., greatly differs from that of the metal.
Therefore, in a method in which carbon black and fine powder of a magnetic metal are secondarily mixed, it is difficult to homogeneously and highly disperse a magnetic metal having a size of the order of micron.
Further, a method in which carbon black and fine powder of a magnetic metal are mixed with a matrix material such as rubber, resin, paint, or ink is also accompanied by separation in the step of mixing, which makes it impossible to stably attain a homogeneous and high degree of dispersion.
Moreover, a high degree of magnetic property cannot be imparted to carbon black when carbon black and the fine powder of a magnetic metal are simply mixed by mechanical means and dispersed in each other, because no fine powder of a magnetic metal is immobilized on carbon black in such a state.
Meanwhile, as a process for preparing carbon black having a magnetic property, Japanese Patent Publication No. 39-25277 discloses a method of supplying an additive containing Fe, Ni, and Co into a reaction zone where carbon black is produced from a hydrocarbon as a starting material. This patent publication describes that the metallic component which has been supplied in the reaction zone where carbon black is produced is converted into a magnetic nucleus, which is in turn uniformly and closely bonded to carbon black present in the nascent state to form a product having a permanent magnetic property.
Further, this patent publication describes that the magnetic nucleus has a size similar to that of carbon black particles and is possibly a metal having a unique physical shape which is sufficiently and closely mingled with or bonded to carbon black particles.
Moreover, this patent publication describes that the resulting carbon black pigment composition contains at least one element selected from among Fe, Co, and Ni in an amount of about 100 to 300,000 parts by weight per 1,000,000 parts by weight of carbon black.
However, no elucidation is made in this prior art on the particle diameters of carbon black and magnetic fine particles and the relationship between these particle diameters and the magnetic property which the magnetic carbon black has.